It is known to mount sample tubes in a tube holder in an analyzer, the holder having a well for each tube. Examples are shown in U.S. Pat. No. 3,680,967 and in EPA 471980 (Docket 60,752). In the former, pairs of spring fingers in each well are used to grip a tube, but no attempt is made to determine the tube diameter, since apparently only one diameter is contemplated. In the latter, varying tube diameters are in fact sensed, in an effective and accurate manner. However, the sensor requires a large, spring biased probe, FIG. 3, which has the disadvantage of excessive size. Still further, the sensor pushes the tube to the inside of the holder as part of the sensing process. Although the sensor determines that in fact the tube center has become displaced from the center of the well, this still requires the analyzer to make an adjustment in the vertical plane, before descending to sample the tube, to avoid the aspirator contacting the tube sidewall.
Therefore, prior to this invention there has been a need to provide a tube-sensing apparatus that is less expensive and less substantial than those required heretofore, which automatically centers the tube with the center of the well, no matter what the tube size is.